Method of forming foam moldings having varied density regions

ABSTRACT

Moldings made from polyolefin particle foams having a mean density of from 0.015 to 0.100 g/cm 3  and having at least two different regions whose densities differ by from 0.005 to 0.030 g/cm 3  have a transition zone between the different density regions having a thickness of not more than 25 mm. A method of forming the moldings, using removable mold dividing walls, is described.

This is a divisional of application Ser. No. 07/809,373, filed Dec. 18,1991, now U.S. Pat. No. 5,164,257.

BACKGROUND OF THE INVENTION

The present invention relates to a polyolefin particle foam moldinghaving various density regions.

Moldings made from polyolefin particle foams have achieved considerableimportance as shock-absorbing packaging and packaging parts and asshock-absorbing parts in the strengthening of motor vehicles, forexample as the core of bumpers, consoles, dashboards or the like. Inaccordance with the desired mechanical properties of the molding, theindividual moldings vary in density and thickness.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide moldings which havezones of various ability to withstand mechanical load. Obvious materialsfor this purpose were polyolefin particle foams having various densityregions. It proved to be too technically complicated to bond individualparts of different density, and welding individual parts of this typedid not give usable moldings either.

We have found that, surprisingly, this object is achieved by moldingsproduced, in a simple manner, in a mold having removable dividing walls.

The present invention accordingly provides a polyolefin particle foammolding having a mean density of from 0.015 to 0.100 g/cm³ and having atleast two different regions whose densities differ by from 0.005 to0.030 g/cm³, the transition zone between the different density regionshaving a thickness of not more than 25 mm.

The present invention furthermore provides a process for the productionof moldings of this type, which comprises introducing fillings ofvarious density made from polyolefin foam particles into the individualchambers of a mold divided into at least two chambers by removabledividing walls, removing the dividing walls, and heating the particlesby passing steam or hot air into the mold so that they soften, expandand weld to one another.

The present invention furthermore provides a mold for carrying out thisprocess, wherein the cavity of the mold is divided into at least twochambers by dividing walls, the chambers being connected via supplylines to storage tanks for polyolefin foam particles of the same ordifferent density, and the dividing walls being designed as retractablecores connected to pressure cylinders via rods.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates a schematic diagram of an apparatus used to molda foamed composite article according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The moldings according to the invention essentially comprise a particlefoam based on a polyolefin. Examples of suitable polyolefins arehomopolymers and copolymers of ethylene and propylene. Preference isgiven to propylene copolymers having a melting point of from 125° to155° C., preferably from 130° to 150° C., in particular from 140° to150° C. The melting point given here is the maximum determined by theDSC method on the second melting of a sample (crystallite meltingpoint). Copolymers of propylene containing from 1 to 30% by weight, inparticular from 1 to 6% by weight, of ethylene and/or a C₄ - to C₆-α-olefin are particularly suitable.

The foam is a particle foam, as obtainable by welding foam particles,usually having a mean diameter of from 2 to 8 mm, preferably from 3 to 6mm. The mean density of the foam, i.e. the density averaged over all thedensity regions of the molding, is from 0.015 to 0.100 g/cm³, preferablyfrom 0.020 to 0.090 g/cm³, in particular from 0.030 to 0.080 g/cm³.

The molding has regions of different density, with the zones which canwithstand higher load having higher density. The density difference isgenerally from 0.005 to 0.030 g/cm³, in particular from 0.010 to 0.025g/cm³. The transition zone between the various density regions isrelatively narrow and has a thickness of not more than 25 mm. Ingeneral, the thickness of this zone corresponds to 1 to 4 times thediameter of the foam particles used.

The foam may also contain, in the individual regions, conventionaladditives such as dyes, pigments, fillers, reinforcing substances,flameproofing agents, release agents, antistatics, stabilizers and otherassistants in effective amounts.

The moldings are preferably used in motor vehicle manufacture, forexample as door panels, dashboards, consoles, sun visors, bumpers andthe like.

The moldings according to the invention are advantageously produced, asshown in the drawing, in a mold whose cavity is divided into at leasttwo chambers 2, 2a by removable dividing walls 3. Foam particle fillingsof different density are introduced into the chambers (2, 2a), inaccordance with the requirements for the moldings. The different fillingdensities can be produced either by using particles of different densityin the individual chambers or by using particles of the same density,but introducing different amounts per volume unit into the individualchambers. In the example shown in the drawing, the chambers 2, 2a areinitially separated from one another by the dividing walls 3 and arefilled with foam particles of different density from the storage tanks7, 8 via the supply lines 5, 6. The filling of the chambers is generallyeffected by compressed air, the pressure used, for example an excesspressure of from 0.5 to 5 bar, causing a certain compression of theparticles. By varying the pressure or by subsequent compression,different fill amounts per volume unit can easily be introduced into thechambers. After decompression, the particles attempt to expand again andpack against one another in such a manner that virtually no movement ofmaterials in the mold takes place when the dividing walls are withdrawn.After the filling operation, the dividing walls 3 are removed from themold by means of the rods 9 and the pressure cylinders 4, and the moldis heated using steam or hot air so that the foam particles soften,expand and weld to one another. After subsequent cooling, the moldingcan be removed from the mold.

We claim:
 1. A process for the production of a molding having a meandensity of from 0.015 to 0.100 g/cm³ and having at least two differentregions whose densities differ by from 0.005 to 0.030 g/cm³, thetransition zone between the different density regions having a thicknessof not more than 25 mm, which process comprises introducing polyolefinfoam particles having substantially the same density into individualchambers of a mold divided into at least two chambers by removabledividing walls, said chambers being filled at different pressures orapplying different degrees of compression to the particles after thechambers have been filled, thereby providing regions of differentdensity, removing the divided walls, and heating the particles bypassing steam or hot air into the mold so that they soften, expand andweld to one another.
 2. A process for the production of a molding asdefined in claim 1, wherein the molding comprises a particle foam basedon a propylene copolymer having a melting point of from 125° to 155° C.3. The process of claim 1 wherein the chambers are filled with particlesat different pressures.
 4. The process of claim 1, wherein the particlesin the different chambers are subjected to different degrees ofcompression.